The present invention relates to semiconductor pressure detecting devices and, in particular, to a semiconductor pressure detecting device which uses as a detecting element a semiconductor sensor element capable of detecting strain and/or stress that occurs to a thin-walled pressure-receiving portion.
As a kind of pressure detecting device, there has conventionally been known one which uses as a detecting element a semiconductor sensor element (hereinafter, abbreviated simply as sensor element when appropriate) capable of detecting strain and/or stress that occurs when pressure acts on a thin-walled diaphragm-like pressure-receiving portion by making use of the piezoresistance effect of semiconductor (for example, see Japanese Patent Laid-Open Publication HEI 9-101219).
With the use of such a type of sensor element, magnitude and/or change of the pressure acting on the pressure-receiving portion can be detected as magnitude and/or change of strain and/or stress with high accuracy, and then converted into an electric signal and outputted as such.
FIG. 4 is an explanatory view in longitudinal section showing the basic structure of a semiconductor pressure detecting device (hereinafter, abbreviated simply as pressure detecting device or as device when appropriate) according to a prior art example.
As shown in this figure, a pressure detecting device 101 according to this prior art example comprises a semiconductor sensor element 102 made of silicon (Si) single crystal and having a thin-walled diaphragm-like pressure-receiving portion 102a in central part, a silicon pedestal seat 103 for bonding and supporting the sensor element 102, and a metallic base member 104 for bonding and supporting the pedestal seat 103. On the counter-pedestal seat side of the base member 104, a pressure introducing pipe 104p is provided integrally.
The base member 104 has a body portion 104a for bonding and supporting the pedestal seat 103 on its upper face side, and a thin-walled flange portion 104f provided on the periphery of the body portion 104a. This flange portion 104f is formed integrally with the body portion 104a by compression molding the peripheral part of the body portion 104a with a press. To the flange portion 104f of this base member 104, a flange portion 111f provided on the periphery of an opening end of a metallic cap 111 is bonded. On the bonding surface of the flange portion 111f of this cap 111, a projection 111p of, for example, annular shape is formed. By fusing this projection 111p, projection welding is performed so that the two flange portions 104f and 111f are bonded to each other.
The pedestal seat 103 comprises a pair of fitting plates 103a, 103b opposed to each other with an annular groove 103g interposed therebetween. A fitting base portion 102b of the sensor element 102 is bonded to the upper face of the upper fitting plate 103b, while the lower face of the lower fitting plate 103a is bonded to the upper face of the body portion 104a of the base member 104.
In addition, to the body portion 104a of the base member 104, a plurality of lead wires 113 are fixed so as to pass therethrough along its thicknesswise direction. Each lead wire 113 is electrically connected to the sensor element 102 via a wire 112 made of, for example, gold (Au).
The outer surface of this sensor element 102 including the pressure-receiving portion 102a is covered with a coating gel layer 106 for use of surface protection after the wiring of the wires 112.
The sensor element 102 and the pedestal seat 103, as well as the pedestal seat 103 and the base member 104 are bonded together, respectively, by the so-called die bonding process so as to be sealed airtight and fluid-tight. As a result of this, an inner passage 104h (pressure fluid introducing hole) which passes through the base member 104 and its pressure introducing pipe 104p, and a pressure fluid introducing hole 103h which is provided in central part of the pedestal seat 103 so as to pass through the pedestal seat 103 are communicated with each other so that pressure fluid is introduced to a pressure chamber 109 formed between the inner wall of the sensor element 102 including the pressure-receiving portion 102a and the upper face of the pedestal seat 103.
In addition, a space defined by an outer surface of a unit body comprising the sensor element 102, the pedestal seat 103 and the base member 104, and by an inner wall surface of the cap 111 constitutes a vacuum chamber 110.
In such a pressure detecting device 101 as shown above, when the pedestal seat 103 and the base member 104 are bonded together by die bonding process, there occurs residual stress due to heat during the die bonding process because of a large difference in linear expansion coefficient between the materials of the pedestal seat 103, which is made of silicon, and the base member 104, which is made of metal.
Also, in such a pressure detecting device 101 as shown above, the flange portion 111f of the cap 111 and the flange portion 104f of the base member 104 are bonded together by projection welding in the final assembly step. During this welding, stress would occur to the surface of the body portion 104a of the base member 104.
For this reason, during the welding of the cap 111 and the base member 104, or upon occurrence of, for example, an impact applied to the pressure detecting device 101 after the welding, there would occur problems such as occurrence of a crack Cr (see broken curved line in FIG. 4) to the pedestal seat 103 made of silicon, which would cause the degree of vacuum of the vacuum chamber 110 to lower, or even without the occurrence of the crack Cr, variations of the pressure characteristic of the sensor element 102 by an effect of welding strain, which would cause the detection accuracy to deteriorate.
The residual stress that occurs during the die bonding process between the pedestal seat 103 and the base member 104 as well as the stress that occurs during the welding process between the flange portions 104f and 111f of the base member 104 and the cap 111 increases more and more with increasing size of the pedestal seat 103 (therefore, increasing area of a bonding surface 103f of the pedestal seat 103 to the base member 104), and with increasing size of the base member 104.
In the semiconductor pressure detecting device 101 as described above, in addition to the semiconductor sensor element 102 having a gauge resistor, various peripheral circuits such as a resistor circuit for adjusting electrical characteristics of the sensor element 102 are required. In recent years, however, there is a tendency that these peripheral circuits and the like are incorporated and integrated in the sensor element itself. In this case, the size of the semiconductor sensor element (particularly, its planar size) becomes larger than when the peripheral circuits and the like are formed on an additional board, so that the size of components (pedestal seat and base member) that support the sensor element also becomes larger inevitably.
Therefore, when the peripheral circuits are incorporated in the semiconductor sensor element, in which case the planar size of the pedestal seat and the base member has been increased resultantly, the aforementioned problems of residual stress and stress due to welding strain would occur more noticeably.
The above-mentioned publication (Japanese Patent Laid-Open Publication HEI 9-101219) has disclosed that a recessed portion is formed at a base-member-side end portion of the pressure fluid introducing hole of the pedestal seat so as to relax the thermal stress that the pedestal seat receives from the base member due to the difference in thermal expansion coefficient between the pedestal seat and the base member, by which reduction in measurement error due to the thermal stress is intended. However, the publication has no particular discussions on the problem of residual stress that occurs during the die bonding process between the pedestal seat 103 and the base member 104, or the problem of stress that occurs during the welding process between the base member 104 and the cap 111, neither does it cover countermeasures for those problems.
The present invention has been accomplished in view of these and other technical problems. An object of the present invention is therefore to provide a semiconductor pressure detecting device which can effectively reduce the residual stress that occurs due to the die bonding between pedestal seat and base member and the stress that occurs due to the welding between base member and cap.
In order to achieve the above object, according to a first aspect of the present invention, there is provided a semiconductor pressure detecting device comprising a semiconductor sensor element capable of detecting strain and/or stress that occurs to a thin-walled pressure-receiving portion, a pedestal seat for bonding and supporting the semiconductor sensor element, a base member for bonding and supporting the pedestal seat, and a cap member which is bonded to an outer peripheral portion, or its proximity, of the base member to cover the base member, the pedestal seat and the semiconductor sensor element, wherein pressure fluid is introduced to a pressure chamber formed between an inner wall including the pressure-receiving portion of the semiconductor sensor element and a sensor-element supporting surface of the pedestal seat via pressure fluid introducing holes provided in the base member and the pedestal seat, respectively, so that fluid pressure acting on the pressure chamber is detected by the semiconductor sensor element, and wherein outer peripheral configuration of a bonding surface of the pedestal seat to the base member is rectangular or generally rectangular shaped, and bonding length in a direction of a diagonal line of the rectangular shape is set based on a prescribed maximum operating pressure of the semiconductor pressure detecting device, conditions for generation of residual strain during a bonding process between the pedestal seat and the base member, and conditions for generation of strain during a bonding process of the cap to the base member.
Also, according to a second aspect of the present invention, in the semiconductor pressure detecting device, a hollow portion including part of the pressure fluid introducing hole is formed at the bonding portion between the pedestal seat and the base member, and the bonding length in the direction of the diagonal line is set by setting a planar size of the hollow portion at the bonding surface.
Further, according to a third aspect of the present invention, in the semiconductor pressure detecting device according to the second aspect of the invention, the hollow portion is formed in the pedestal seat.
Still further, according to a fourth aspect of the present invention, in the semiconductor pressure detecting device according to the second aspect of the invention, the hollow portion is formed in the base member.
In the first aspect of the present invention, the outer peripheral configuration of the bonding surface of the pedestal seat to the base member is rectangular or generally rectangular shaped and, with respect to this bonding surface, the bonding length in the direction of the diagonal line of the rectangular shape is set based on the prescribed maximum operating pressure of the semiconductor pressure detecting device, conditions for generation of residual strain during the bonding between the pedestal seat and the base member, and conditions for generation of strain during the bonding of the cap member to the base member. Accordingly, the bonding length in the direction of the diagonal line of the bonding surface can be set short within such a range that the residual stress that occurs due to the bonding between the pedestal seat and the base member and the stress that occurs due to the bonding of the cap member to the base member with application of the prescribed maximum operating pressure do not exceed the bonding strength between the pedestal seat and the base member.
That is, the bonding area between the pedestal seat and the base member can be reduced by setting short the longest portion of the bonding surface therebetween, so that residual stress that occurs due to the bonding between the pedestal seat and the base member can be reduced and that the effect on the pedestal seat by strain that occurs due to the bonding between the cap member and the base member can be suppressed. As a result, during the bonding between the cap member and the base member, or upon occurrence of, for example, an impact applied to the pressure detecting device after the bonding, such malfunctions as occurrence of cracks in the pedestal seat, which would cause the degree of vacuum of the vacuum chamber to lower, or even without the occurrence of the cracks, variations of the pressure characteristic of the sensor element by an effect of bonding strain, which would cause the detection accuracy to deteriorate, can be suppressed.
Also, according to the second aspect of the present invention, basically, the same effects as in the first aspect of the invention can be produced. In particular, the hollow portion including part of the pressure fluid introducing hole is formed at the bonding portion between the pedestal seat and the base member, and the bonding length in the direction of the diagonal line is set by setting the planar size of the hollow portion at the bonding surface, so that the setting of the bonding length can be achieved accurately. Also, since the hollow portion is formed in central part of the bonding portion between the pedestal seat and the base member, the effect of any strain that occurs due to the bonding between the cap member and the base member can be reduced reliably.
Further, according to the third aspect of the present invention, basically, the same effects as in the second aspect of the invention can be produced. In particular, since the hollow portion is formed in the pedestal seat, the hollow portion can be formed simply and reliably by working this pedestal seat.
Still further, according to the fourth aspect of the present invention, basically, the same effects as in the second aspect of the invention can be produced. In particular, since the hollow portion is formed in the base member, the hollow portion can be formed simply and reliably by working this base member.